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1. Introduction - Firenze University Press

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Table <strong>1.</strong> Characteristic quantities for investigated air separation unit (ASU)<br />

Name Symbol Value Unit<br />

Ambient pressure<br />

Ambient temperature<br />

Membrane working temperature<br />

pot<br />

tot<br />

tmem<br />

10<strong>1.</strong>3 kPa<br />

o<br />

20 C<br />

o<br />

750;850 C<br />

Stream of separated oxygen m O2<br />

107.56 kg/s<br />

Oxygen recovery rate R 40÷100 %<br />

Compressor pressure ratio k 2÷30 -<br />

Compressor isentropic efficiency iS 0.88 -<br />

Expander isentropic efficiency iT 0.9 -<br />

Generator efficiency g 0.99 -<br />

It was assumed for the calculations that the air taken from environment is a dry gas consisting of<br />

21% oxygen and 79% nitrogen (volumetric composition).<br />

The characteristic quantities gathered in Table 1 were used for computations performed on a "fourend"<br />

membrane air separation unit model, made in GateCycle TM software. The built-in components<br />

were used to build the air separation unit model. The quantity that determinates the value of the<br />

mass flow rate in the entire ASU model is a mass flow rate of oxygen. This mass flow rate is<br />

approximately the same as the oxygen mass flow rate feeding an oxy boiler working in a 460 MW<br />

power plant. It was assumed that through the membrane flows pure oxygen.<br />

The structure of an autonomous gas turbine as opposed to a structure of ASU shown in Figure 2<br />

does not contain a "four-end" type membrane. In the autonomous gas turbine model the assumption<br />

concerning a compressor, expander and air heater are the same as in the ASU model. The air mass<br />

flow rate in both models are the same, the difference is only in the mass flow rate and composition<br />

of gas flowing into the expander. In the ASU model the composition and mass flow rate of the gas<br />

is different from air because some oxygen is separated from air in the membrane. In the<br />

autonomous gas turbine model the mass flow rate and composition of gas flowing into the expander<br />

is the same as mass flow rate and composition of air leaving the air heater.<br />

3. The results of calculations of air separation unit and<br />

autonomous gas turbine<br />

The air mass flow rate depends on the separated in membrane oxygen mass flow rate ( m O2 ), oxygen<br />

recovery rate (R) and mass content of oxygen in the air ( g O2air<br />

). The relationship between these<br />

quantities is as follows:<br />

m<br />

O2 1a , (2)<br />

R<br />

gO2air<br />

m<br />

Next the air is compressed by the compressor. Effective power required to drive the compressor<br />

depends on the air mass flow rate ( 1a m ), the air temperature ( 1a T ), the average specific heat ( ~c p ),<br />

K<br />

1<br />

the compressor pressure ratio ( K ), the heat capacity ratio contained in the factor ( K <br />

),<br />

K<br />

the compressor isentropic efficiency ( iK ) and the compressor mechanical efficiency ( mK ). The<br />

equation showing the relationship between these quantities is as follows:<br />

38

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